Switching between subjective and objective modes is the essence of the scientific modus operandi. Not many people seem to appreciate that. Science is all about riding two horses, maybe not in concert, but certainly alternately - and knowing when to switch from one to the other.

This is just a holding post. I'll insert the comments I've been posting on the Guardian and Independent later. They will show an evolution in my thinking, informed (or misinformed|) by the Indy quoting from an US inspector's report that the site DID have ammonium nitrate as well as anhydrous ammonia.

As the search and rescue operation continued, agents from the federal
Bureau of Alcohol, Tobacco, Firearms and Explosives scoured the scene
for clues to the cause of the blast, which officials said had destroyed
about 50 homes. A recent report submitted to the Texas Department of
State Health Services suggested the facility contained a stockpile of up
to 270 tonnes of ammonium nitrate, and 100,000lb of liquid ammonia.

(ed: normally, when you link to a report in a newspaper, that report stays the same. The UK's Independent is a notable exception. Articles are not only edited but can be completely rewritten - while still appearing under the same URL. Let the internet surfer - or in this case blogger - beware)

Here's the way my thoughts are going. First, one does not expect liquid ammonia (NH3) to behave like rocket fuel when tanks are ruptured by fire. Escape of ammonia gas yes. Fireball, no. Surely there would be well known precedents if that were the case, with regulations siting the tanks far from population centres. Secondly, one does on the other hand have precedents too numerous to mention of ammonium nitrate producing devastating explosions that have flattened entire communities and killed scores, sometimes hundreds. But there is scarcely any mention of ammonium nitrate as a causative agent in connection with West-comma- Texas near Waco.(ed: the above sentence was true at the time of writing; we have since learned that some 270 tonnes (yes!) was stored on that site, in breach of National Homeland Security regs).

Here's my hunch. There was BOTH liquid ammonia AND ammonium nitrate on site, and the heat of the initial fire (wooden pallets?) was sufficient to melt the ammonium nitrate and then rupture the ammonia storage tanks.

When molten ammonium nitrate decomposes it produces nitrous oxide, N2O, a gas with similar oxidising properties to oxygen, able to support combustion ( even relights a glowing splint as I recall, the "standard" test for oxygen).

Ammonia gas is reluctant to burn in air, unless introduced into an existing flame, and raised to its air/ammonia ignition temperature of close on 1000 degrees C.

But suppose there had been a mixture of ammonia gas and nitrous oxide, the latter behaving like oxygen. Might that have not been the reason for ammonia behaving like rocket fuel?

@Daveinireland - All we know for certain is that liquified, pressurized anhydrous ammonia was on site.
Theoretically
it could have produced the fireball if the ignition temperature of
air/ammonia gas mixtures of close on 1000 degrees C had been reached.
But if the accident investigators know their explosives chemistry, they
will be looking for traces of solid ammonium nitrate and/or for any
evidence that the solid was either shipped in, or made on site by
reacting ammonia and nitric acid.
Methinks we would have had
more precedents in the past for what happened at West, Texas if
anhydrous liquid ammonia really were given to behaving like rocket fuel
when there's a nearby fire.

The short science lesson would have been useful, had it been correct.
Ammonia is a gas at ordinary temperature and pressure. It does not
form explosive mixtures with air, but will combust to nitrogen and steam
if introduced into an existing flame, the latter being maintained. In
other words, ammonia and air alone will not produce a conflagration and
certainly not an explosion. Were that the case, there would have been
hundreds of explosions from leaking refrigeration plants.
It can be liquified under pressure alone (normal temp and pressure).
The only significance of that -33 degrees C is that any leak of liquid
ammonia would result in the liquid boiling at the temperature, and
quickly changing to highly toxic gas.
As for the fireball, it's hard to see how that could be due to liquid
ammonia, even if the tanks ruptured, although the gas might have
assisted an existing fire, as mentioned above. What is far more probable
is that solid ammonium nitrate was also on site, and that as we know
can be a very dangerous and unpredictable substance if exposed to heat
alone. Leaving aside specialized detonation to produce instant
decomposition with powerful shock wave ("fertilizer bomb") ammonium
nitrate produces vast amounts of gas when it thermally decomposes to
nitrous oxide and steam, both gaseous, and the only products. One
imagines that it was a runaway decomposition that produced the vast
fireball that was for all intents and purposes an explosion, even if the
chemical purists prefer to describe it as a rapid decomposition rather
than explosion.
It's bad enough that liquid ammonia was stored so close to a
community - but if reports that ammonium nitrate was also there are
correct, then words simply fail one. Have they not read their history
books? Many, probably most, of the worst most devastating chemical
disasters in history in which hundreds have lost their lives have
involved harmless-looking, salt-like ammonium nitrate.
NH4NO3 (solid) -> N2O (gas) + 2H2O (steam)
That is the chemical equation for a potential time bomb - especially
if fire breaks out in the vicinity. All that reaction needs is the right
(wrong) kind of kick start.

"Officials at first suggested the explosion was caused by
the anhydrous ammonia igniting, but it was revealed Thursday through
Texas state records that the plant also possessed 270 tons of ammonium
nitrate, a much more volatile, dry solid, at the end of 2012. Records
also suggested that in 2006, the Environmental Protection Agency (EPA)
issued a $2,300 fine for deficiencies in the plant's risk management
plan."

If there had been even a modest amount of ammonium
nitrate as well as anhydrous ammonia on site, and the two were
pre-heated close to one another, there is an explanation for that
massive fireball.
Ammonium nitrate decomposes to nitrous oxide,
N2O, an oxidant that behaves almost like pure oxygen (relights a glowing
splint). Ammonia, reluctant to burn in air except at high ignition
temperatures (close on a 1000 degrees C) can burn in pure oxygen, so
probably in nitrous oxide too, maybe at much lower temperatures than
1000 degrees.

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Colin Berry, aka sciencebod, is a retired PhD researcher/teacher/academic who has worked in industry, medical schools, schools, food and biomedical research (mainly in the UK, but also in W.Africa and the United States). He's best known for his work on RESISTANT STARCH, recently described as "the trendiest form of dietary fibre".
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Acknowledgment

What's the latest on the LHC?

LHC gets warning system upgrade : BBC 28 September 2009

Self-organization

From wiki entry on SELF ORGANIZATION: "As a result, processes considered part of thermodynamically open systems, such as biological processes that are constantly receiving, transforming and dissipating chemical energy (and even the earth itself which is constantly receiving and dissipating solar energy), can and do exhibit properties of self organization far from thermodynamic equilibrium."

How far away should your off-licence be for a bottle of wine to be energy-neutral?

What do these two have in common?

Answer: both arrived in this world about the same time. Sir Isaac Newton was born on 4th Jan 1643 (new style*). The Taj Mahal had a 20 year gestation period, centred on approximately the same year. Click on piccy for an older post .* Or Christmas Day, 1642, depending which dating system one uses.

Is interstellar space travel feasible?

The nearest star (more correctly, star system, since it's 3 stars, a binary and a smaller satellite star) is Alpha Centauri. The average distance from Earth is 4.3 light years. Suppose technology allows us one day to achieve an interstellar cruising speed of half the speed of light. A comfortable acceleration of g (simulating Earth's gravity) would take a year, with another year to slow down comfortably. The entire journey from Earth would take a minimum of 10 years approximately. Having arrived at one's destination, it would take 4.3 years to send a radio postcard (" Hello Mum and Dad. Have arrived safely, and am now looking for a habitable planet. Am hoping it's hiding behind Proxima. Have looked everywhere else... Would die for some Cheddar cheese... ")

Watch this space

It's a cheap and cheerful form of transcendental meditation.(experimenting with settings, actually)

What causes weather?

Could you answer that question in just 7 words, ie " weather is due to...? Need some help, " Weather is due to t- - u - - - - - - h - - - - - - o - t - - E- - - -'s s - - - - - - ." The National Curriculum (England and Wales) does have its uses, but there are many more such simple principles, expressed in a minimum of words, that could be usefully incorporated.

"Had there been a Beginning (there wasn't, as it happens), there would initially have been complete Nothingness. But just as Nature abhors a vacuum, it's totally gutted at the thought of Nothingness. I mean to say - how far does Nothingness extend, assuming it has one of more dimensions? It can't extend for an infinite distance, since that would be a physical impossibility. Nothingness, to avoid having infinite reach, coils up on itself to acquire finite dimensions. In so doing, it becomes Somethingness, which has a spring-like potential energy - the total energy in fact of the Universe.

From that potential energy, present in what we now call space, or space-time, which is anything but empty, is spawned all sub-atomic particles - both matter and antimatter. When those particles collide, they mutually annihilate to create photons.

The reverse can also happen under extreme conditions - two photons can collide to create matter and anti-matter. It is potential energy in the spring-coiled Universe that is our "Dark Energy. It may or may not have mass depending on conditions.

A moment when it has no mass is the instant of the Big Bang. Let me briefly explain. An oscillating universe switches between Big Bang and Big Crunch. With the latter gravitation pulls everything into a super blackhole which then becomes a singularity - a massively dense point in space-time.

What prevents it becoming infinitely small - a physical impossibility? Answer: friction. As the sub-atomic plasma contracts and grinds, heat is generated which cannot escape - being a black hole. The temperature rises, ie particles in the plasma move faster and faster. When they reach their maximum velocity - the speed of light- all particles are suddenly transformed into photons, which as we know have no true mass(at least, no rest mass: any mass they have is purely relativistic due to their speed).

Once the entire Universe is a super-concentration of photons, all the gravitational forces in the singularity collapse to zero, or nearly so, and the entire thing blows apart - a new Big Bang, to create yet another cycle (inflation, Big Crunch, implosion etc). The Big Bang creates not just sub-atomic particles - from photon-photon collisions, but space-time itself. To reiterate: that space-time is always suffused with the stored potential energy of our curled-up dimensions (Dark Energy)."